There are many applications which prescribe, in general terms, that a fastener be driven a fixed distance into a workpiece. One such application arises when securing sections of drywall sheathing material to wall studs using drywall screws. In this application, it is desired that each drywall screw be driven until the screw head is substantially flush with the drywall surface or the head is a predetermined distance below the drywall surface. If the screw is under-driven, the screw head protrudes from the drywall surface. This yields a undesirable bulge in the wall at the location of the screw. If the screw is over-driven, the screw head penetrates into the drywall. An over-driven screw can cause damage to the drywall, such as cracking.
To facilitate rapid fastening of drywall sections to studs, a handheld power screwdriver is employed. Although a standard power screwdriver can increase the rate at which drywall screws are fastened, it does not include means for driving the screw until the head is flush with the surface.
The need to control the depth at which a fastener is driven into a workpiece precipitated the advent of a depth adjusting device for a power screwdriver. Many of these depth adjusting devices comprise a front end which contacts, or engages, the workpiece. U.S. Pat. No. 3,527,273 to Falter discloses a tool having such a depth adjusting device in which a clutch disengages to terminate rotation once a bit reaches a predetermined position with respect to a tool housing. U.S. Pat. No. 4,592,257 to Durr discloses a depth adjusting device having a depth stop that is robust to operational disturbances.
Although the apparatuses disclosed in the cited references include means for adjusting the driven depth of a fastener, they do not include means for gauging the depth that will result based on an adjustment. Therefore, the process of adjusting the stop depth to a desired location is iterative. One must first make an initial depth adjustment followed by driving a first fastener. Next, a second depth adjustment is made based on an eyeballing of the depth of the first fastener within the workpiece followed by driving a second fastener. This process of adjusting the depth and driving fasteners is repeated until the desired depth is attained.